From 6b908c1bb18d3a7eb04e2fa6e979176c32eff9e0 Mon Sep 17 00:00:00 2001
From: Warren Weckesser <warren.weckesser@gmail.com>
Date: Wed, 18 Nov 2015 13:55:57 -0500
Subject: [PATCH] BUG: io: imread in the PIL plugin drops indexed PNG alpha
 channel.

When using the PIL plugin to read an indexed PNG file that has
an alpha channel, the alpha channel would be lost.
---
 skimage/data/foo3x5x4indexed.png  | Bin 0 -> 116 bytes
 skimage/io/_plugins/pil_plugin.py |   5 ++++-
 skimage/io/tests/test_pil.py      |  22 ++++++++++++++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 skimage/data/foo3x5x4indexed.png

diff --git a/skimage/data/foo3x5x4indexed.png b/skimage/data/foo3x5x4indexed.png
new file mode 100644
index 0000000000000000000000000000000000000000..cc969d03dd263fde0293045012129ad1e01ea824
GIT binary patch
literal 116
zcmeAS@N?(olHy`uVBq!ia0vp^tU%1n!3-pGUYMQ&Qak}ZA+8Jz{~6@}*E9S-aNz$2
z)p@}{ah8%GzhH*{{~6}#uLo%o_H=O!shE?Tz{JRwmd2)#z{1GD@Qp!iqy9%JpfrQ0
LtDnm{r-UW|Gdmrm

literal 0
HcmV?d00001

diff --git a/skimage/io/_plugins/pil_plugin.py b/skimage/io/_plugins/pil_plugin.py
index 7cb8e5c1..59811dd2 100644
--- a/skimage/io/_plugins/pil_plugin.py
+++ b/skimage/io/_plugins/pil_plugin.py
@@ -85,7 +85,10 @@ def pil_to_ndarray(im, dtype=None, img_num=None):
             if grayscale:
                 frame = im.convert('L')
             else:
-                frame = im.convert('RGB')
+                if im.format == 'PNG' and 'transparency' in im.info:
+                    frame = im.convert('RGBA')
+                else:
+                    frame = im.convert('RGB')
 
         elif im.mode == '1':
             frame = im.convert('L')
diff --git a/skimage/io/tests/test_pil.py b/skimage/io/tests/test_pil.py
index ad3870a6..ee962906 100644
--- a/skimage/io/tests/test_pil.py
+++ b/skimage/io/tests/test_pil.py
@@ -84,6 +84,28 @@ def test_imread_palette():
     assert img.ndim == 3
 
 
+def test_imread_index_png_with_alpha():
+    # The file `foo3x5x4indexed.png` was created with this array
+    # (3x5 is (height)x(width)):
+    data = np.array([[[127, 0, 255, 255],
+                      [127, 0, 255, 255],
+                      [127, 0, 255, 255],
+                      [127, 0, 255, 255],
+                      [127, 0, 255, 255]],
+                     [[192, 192, 255, 0],
+                      [192, 192, 255, 0],
+                      [0, 0, 255, 0],
+                      [0, 0, 255, 0],
+                      [0, 0, 255, 0]],
+                     [[0, 31, 255, 255],
+                      [0, 31, 255, 255],
+                      [0, 31, 255, 255],
+                      [0, 31, 255, 255],
+                      [0, 31, 255, 255]]], dtype=np.uint8)
+    img = imread(os.path.join(data_dir, 'foo3x5x4indexed.png'))
+    assert_array_equal(img, data)
+
+
 def test_palette_is_gray():
     gray = Image.open(os.path.join(data_dir, 'palette_gray.png'))
     assert _palette_is_grayscale(gray)